1. Field of the Invention
The present invention relates to a light emitting device containing a light emitting element and a process for producing such a light emitting device, where the light emitting element is constituted by a transparent substrate and a stack of GaN-based compound semiconductor layers formed on the transparent substrate.
2. Description of the Related Art
Conventionally, light emitting devices for light irradiation are used in various apparatuses (e.g., optical reading apparatuses such as facsimile apparatuses and image scanners, and fixing devices in color thermal printers). Various light emitting devices which use light emitting diodes (LEDs) as light sources have been proposed. The LEDs are small in size and relatively stable.
For example, in the fixing devices in color thermal printers, ultraviolet light is applied to thermal color-forming layers in color thermal recording sheets in order to fix recording images thermally recorded in the color thermal recording sheets. However, the emission efficiencies (optical output/current) of the conventional LEDs having emission wavelengths suitable for fixing thermal color-forming layers for yellow and magenta are low, and energy conversion efficiencies (optical output/input power) are also low. Therefore, the intensities of illumination are too small.
In order to obtain sufficient intensity of illumination, increase in packaging density of LEDs is considered. However, when the packaging density is high, it is difficult to sufficiently dissipate heat generated by the LEDs. Therefore, the element temperature rises, and the emission efficiency drops. Thus, the challenge is to increase the utilization efficiency of light emitted from each LED.
Incidentally, in some LEDs, a light emitting element constituted by a transparent substrate and a stack of GaN-based compound semiconductor layers formed on the transparent substrate is mounted on a lead frame or a printed circuit board so that the transparent substrate is located on the opposite side to the lead frame or printed circuit. In such LEDs, light emitted from a light-emission surface of the stack of GaN-based compound semiconductor layers and is incident on a light-exit surface (upper surface) of the transparent substrate at an angle greater than a critical angle is totally reflected at the light-exit surface, and directed to a side surface of the transparent substrate. When the light totally reflected at the light-exit surface is incident on the side surface of the transparent substrate at an angle greater than the critical angle, the light is totally reflected at the side surface of the transparent substrate, and returned to the light-exit surface. The light returned to the light-exit surface is absorbed by the light-exit surface, and is therefore not effectively used. Thus, the utilization efficiency of the emitted light decreases.
In order to solve the above problem, Japanese Unexamined Patent Publication No. 6(1994)-244458 discloses an LED in which a side surface of the light emitting element is cut at a bevel from the upper surface of the transparent substrate so that the side surface makes an acute angle with the upper surface, and light reflected at the light-exit surface is incident on the side surface at an angle not greater than the critical angle. In this case, a greater amount of light can go out of the light emitting device and utilized, i.e., the utilization efficiency of emission light can be increased.
However, in the above LED having a side surface beveled at the acute angle, the upper surface area of the transparent substrate is greater than the light-emission surface area of the light emission region, and the upper portion of the LED overhangs in horizontal directions. Therefore, downsizing of the LED in the horizontal directions is difficult. In particular, when a plurality of LEDs are arranged in an array on a circuit board or the like, it is difficult to conserve space.